This SIV vaccine trial was based on a combination immunization protocol with recombinant vaccinia virus containing SIV gp160 or gp120. Booster immunizations were done with either baculovirus-SIV gp120, vaccinia-SIV gp120 or CHO-SIVgp120. Viral challenge was performed with SIVmac239(nef followed by rechallenge with SIVmac251. Six of thirteen (46%) vaccinated animals were long-term survivors and did not develop AIDS. In contrast, all of the seven unvaccinated control animals that were held throughout the study developed AIDS. The surviving vaccinated animals also had stable CD4+ cell numbers and lower plasma viral loads than animals that developed AIDS. Another perspective on the survival data can be seen when comparing the longest-living unvaccinated control with the vaccinated groups, of which five are still alive and appear healthy. Nine of thirteen vaccinees survived longer than the longest-living unvaccinated control. The average survival time for the unvaccinated group was 56 weeks, whereas the vaccinated group with five animals still alive averages >140 weeks of survival. Therefore, it appears that vaccinated animals have a general pattern of increased survival and have very low viral loads and stable CD4 levels.